In the known processes for forming raised contact metallizations, which are also technically described as so-called "bumps", a fundamental differentiation is to be made between mask-oriented processes and processes which are rather to be described as mechanical.
Processes are described as mask-oriented processes where the arrangement and geometric state of the raised contact metallizations are defined by surface masks which only leave free the regions of connection surfaces of a substrate surface which is otherwise covered by the mask. Only then in a second procedural step is the contact material applied to the connection surfaces, which are technically also described as so-called "pads", for example by means of galvanic or chemical separation processes.
In all of the aforementioned, mask oriented processes the selective application of the contact material intended for forming the raised contact metallizations is achieved by masking the substrate surface. To assemble the mask, usually photolithographic processes for structuring a photosensitive lacquer or similar processes are required. In addition, after the formation of the raised contact metallizations the mask must again be removed from the substrate surface. In particular the process expenditure associated with the formation of such a mask is only worthwhile with mass production of substrates with standardized connection-surface distribution.
In contrast, the aforementioned mechanical processes for forming raised contact metallizations have the advantage that a selective formation of raised contact metallizations on the pads is possible without the previous formation of an appropriate surface mask. With such processes the wire connection technology is used which has been developed in itself to make wire connections between connection surfaces of the same substrate or different substrates. In wire connection technology a process technically described as "ball-wedge-process" has become generally accepted, where first of all a ball is formed on a wire end section, drawn from the nose-piece of a bonding capillary, by means of thermal loading of the wire end. This ball is connected to the first connection surface with deformation by means of the nose-piece of the bonding capillary. Subsequently, a wire loop is formed to overcome the distance between the first connection surface and a second connection surface, and finally to make the connection between the wire end section and the second connection surface the wire end section is pressed with deformation against the second connection surface with a connection region of a pressure surface of the capillary nose piece, and with simultaneous separation of the partial wire section is connected to the connection surface. This second making of a connection is technically described as "wedge".
The use of this method known in itself from wire connection technology for the formation of a raised contact metallizations on a substrate connection surface is disclosed in DE- 32 09 242.
In the known process a wire ball formed with thermal loading at the free end of a wire end section drawn from the capillary nose piece is pressed against a substrate connection surface and is connected thereto. The separation of the wire end section drawn from the capillary nose piece to form the raised contact metallizations takes place by forming a weak point at the wire end section drawn from the capillary nose piece and by subsequently pulling at the weak point.
The raised contact metallizations formed with the known process has a substantially circular base because of the basic spherical shape of the wire end section drawn from the capillary nose-piece. This results in a relatively small overlap between the raised contact metallizations and the usually rectangular surface of the connection surface. In addition, the volume of the raised contact metallizations which can be attained with the known process is limited by the size of the contact material ball which was previously formed at the wire end section. The ball size is in turn dependent on the wire diameter, with the result that proceeding from a given wire diameter only raised contact metallizations with a fixed volume can be attained.
The object of the present invention is to suggest a process and a device for forming a raised contact metallization on a substrate connection surface, which make possible an adaptation of the volume of the raised contact metallization to the geometry and size of the connection surface.
The object is achieved with a process having the features of the following described invention.
In the process in accordance with the invention first of all a first connection is made between a free end of the wire end section of a contact material wire and the connection surface in order to form a first connection region, and subsequently a second connection is made between a running end of the wire end section, which is connected to the remaining contact material wire, and the connection surface or a partial region of the wire end section connected to the connection surface in order to form a second connection region, in such a way that between the first and the second connection region a defined length of wire is formed and the connection regions form together with the length of wire a contact material volume, and finally there is a remelting of the contact material volume formed on the connection surface in order to form the raised contact metallization.
Differing from the previously known process disclosed in DE 32 09 242, in addition to the connection regions of the wire end section, the contact material volume available to form the raised contact metallization is defined by the length of wire produced between the connection regions. Because the material volume of the length of wire depends on the length of the length of wire, the volume desired for the formation of the raised contact metallization can be adjusted easily by an appropriate length of the length of wire. This type of volume adjustment is independent of the cross section of the contact metal wire used. It is also possible, by means of an appropriate course of the length of wire, to adapt the latter to a particular extent to the surface geometry of the connection surface.
In this respect, the second connection can be made just as the first connection directly with the connection surface or also with a partial region of the wire end section which is already connected at one end to the connection surface. The second alternative proves advantageous particularly if the connection surface only has a relatively small surface.
In particular for the latter case it also proves advantageous if the second connection of the wire end section with the partial region of the wire end section which is already connected at one end to the connection surface is made in such a way that the partial region corresponds with the first connection region. In this way both connection regions are arranged directly one on top of the other, with the result that an arrangement of the connection regions is created such that a large amount of space is saved and such that the connection regions overlap, whereby--as in the aforementioned cases as well--the material volume required to form the raised contact metallizations is provided substantially by the length of wire formed between the connection regions.
In certain cases, however, it can also prove to be advantageous if the second connection of the wire end section with the partial region of the wire end section which is already connected at one end to the connection surface is made in such a way that the partial region is arranged with clearance from the first connection region. In this respect, the clearance can even be selected such that the second connection region is located outside the connection surface, whereby, nevertheless, a defined length of wire is created. This can be of advantage in particular if very high contact metallizations are to be formed on very small connection surfaces.
In particular with connection regions which lie relatively close to each other or also one on top of the other, a formation of the bonding tool as bonding capillary proves advantageous, the capillary being provided with a capillary nose-piece which has at least one pressure surface for the formation of a connection region at a wire end section drawn from the capillary nose-piece, whereby adjacent to the pressure surface a wire accommodation area is formed which is used for the recessed accommodation of the wire end section in the surface of the capillary nose-piece.
With the wire accommodation area an avoidance area is formed for the length of wire, so that the length of wire can be formed in a substantially undisturbed manner even when the connection regions lie close to each other or one on top of the other.
To be able to form lengths of wire not only in a main orientation it proves advantageous if the capillary nose-piece is provided with another pressure surface, arranged opposite the first one, for the formation of a connection region, and if a wire accommodation area is likewise provided adjacent to this additional pressure surface. In addition, however, the process in accordance with the invention can also be carried out with bonding tools designed as bonding wedges.
In the following the process for forming raised contact metallizations and a suitable device for carrying out the process will be explained by way of example with reference to the drawings.